The present invention relates to microfluidic devices fabricated by surface micromachining, and in particular to electrokinetic pumps and hydraulic actuators formed on a substrate by surface micromachining.
Surface micromachining utilizes conventional integrated circuit (IC) processing steps to form mechanical or electromechanical devices on a substrate (typically silicon) by building up a structure for a particular device layer by layer. Although many different types of electrostatic actuators can be formed by surface micromachining, the force which can be produced by electrostatic actuators is generally limited. Furthermore, electrostatic actuators are generally limited to motion in the plane of the substrate.
Microfluidic devices based on electrokinetic pumping are capable of producing very high hydraulic pressures of up to 2500 psi without any moving parts (see, e.g. U.S. Pat. Nos. 6,013,164 and 6,019,882 to Paul et al). Such electrokinetic microfluidic devices, based on an electroosmotic flow of a fluid through a microchannel produced by an applied electric potential, necessarily require that there be a porous dielectric medium present in the microchannel.
The present invention is a surface-micromachined microfluidic apparatus formed on a substrate using surface-micromachining, with many applications including pumping or pressurizing a fluid, separating different constituents in a fluid, conducting chemical reactions on a micro-scale, and forming hydraulic actuators.
An advantage of the present invention is that surface micromachining processes can be used to form one or more microfluidic devices in a monolithic form on a substrate.
Another advantage of the present invention is that the microfluidic device can be electrically isolated from the substrate to allow a plurality of electrical connections to be made to the microfluidic device, to allow a plurality of microfluidic devices to be formed on the same substrate, or to allow fabrication on an electrically conducting substrate.
Yet another advantage is that, in some preferred embodiments of the present invention, an electroosmotic force can be produced to act upon a fluid without the need for a microporous dielectric medium as has been heretofore required.
Still another advantage of the present invention is that a series of surface micromachining process steps can be used to form an electroosmotic microfluidic devices, an electromagnetic microfluidic device or a combination thereof on the same substrate.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to a surface-micromachined apparatus, comprising a microchannel (also termed herein a microfluidic channel, a fluid-flow channel or simply a channel) formed on a substrate (e.g. comprising silicon) from a plurality of deposited and patterned layers of polycrystalline silicon (also termed polysilicon) and silicon nitride, with the silicon nitride at least partially lining the microchannel, and means for generating an electroosmotic force or electromagnetic field in the microchannel.
In some embodiments of the present invention, the microchannel can include a constricted portion having a lateral dimension smaller than the lateral dimension of the remainder of the microchannel. For example, the constricted portion can have a height that is smaller than the height of the remainder of the microchannel, with the height of the constricted portion generally being in the range of 0.1-1 microns. An overall width of the constricted portion of the microchannel can be in the range of 0.05-10 millimeters depending upon a particular application. The exact shape of the microchannel and the constricted portion thereof is defined by a removable sacrificial material such as silicon dioxide or a silicate glass. The silicon nitride, which at least partially lines the microchannel, can have a thickness, for example, in the range of 0.1-2 microns.
In other embodiments of the present invention, the microchannel can include a plurality of spaced posts extending outward from at least one wall thereof to increase the surface area in the channel, with the posts being lined with the silicon nitride. These spaced posts can be used in embodiments of the present invention wherein the microchannel is unconstricted, or in embodiments wherein the microchannel includes a constricted portion thereof.
The means for generating the electroosmotic force or electromagnetic field in the microchannel in one embodiment of the present invention comprises a plurality of electrodes for generating an electric potential within the microchannel in response to a voltage provided to the electrodes, with the voltage generally being limited to about 1000 volts or less. The electrodes can be formed from any electrically-conductive material such as polycrystalline silicon, a metal or metal alloy, and carbon (e.g. a doped deposited diamond-like form of carbon).
Various configurations of the electrodes are possible. To generate an electroosmotic force, a first electrode can be located proximate to one end of the constricted portion of the microchannel, and a second electrode can be located proximate to the other end of the constricted portion of the microchannel so that an electric field is produced along the length of the constricted portion to produce a force which acts upon a fluid within the constricted portion. One or more additional electrodes can be located in or proximate to the microchannel between the first and second electrodes to aid in generating the electroosmotic force or to effect an electric field separation of constituent components in the fluid. These additional electrodes can be spaced across the width of the constricted portion of the microchannel, along the length of the constricted portion, above and below the constricted portion, or a combination thereof.
The means for generating the electroosmotic force or electromagnetic field in the microchannel in another embodiment of the present invention comprises a coil formed about a portion of the microchannel. The coil, which can have a rectangular cross-section shape with a height that is generally smaller than a width thereof, can be activated by an electrical current to produce the electromagnetic field in the microchannel.
In each of the above embodiments of the present invention, a fluid (e.g. an electrolyte) can be introduced into the microchannel at an entrance port at one end of the microchannel. The fluid is moveable within the microchannel in response to the generated electroosmotic force or the electromagnetic field. In this way, the fluid can be conveyed through the microchannel from the entrance port thereof to an exit port thereof. One or more of the entrance and exit ports can be formed to extend through the thickness of the substrate to a back side thereof where fluidic connections can be made to the substrate.
In some embodiments of the present invention, the fluid flow can be blocked at one end of the microchannel, for example, to form a hydraulic actuator comprising a closed chamber connected to the microchannel, with the chamber having one or more walls thereof that are moveable in response to a change in pressure of the fluid. In this case, the electroosmotic force or electromagnetic field can be used to pressurize the fluid and displace each moveable wall of the chamber. An actuator arm, lever, linkage, compliant mechanism or a combination thereof can be used to transmit the motion of the moveable wall to a load which can be located either in the plane of the substrate or at an angle (e.g. 90xc2x0) to the substrate. In this way, a hydraulic actuator can be formed to provide an actuator force much larger than the force which is possible with a conventional electrostatic actuator.
The present invention also relates to a surface-micromachined apparatus, comprising a microfluidic channel formed on a substrate and defined, at least in part, by a first layer of silicon nitride, and a second layer of silicon nitride overlying the first layer, with the second silicon nitride layer being nonplanar and thereby forming a constricted portion of the channel having a height that is smaller than the height of the remainder of the channel, and means, located within the channel, for generating a flow of a fluid in the channel. The channel can be further defined by at least one layer of polycrystalline silicon overlying the second layer of silicon nitride. When a pair of polycrystalline silicon layers are provided overlying the second silicon nitride layer, the two layers of the polycrystalline silicon can be separated by a third layer of silicon nitride.
The flow generating means can comprise a first plurality of electrodes disposed in the channel, with the first plurality of electrodes being spaced about the length of the channel to generate an electroosmotic force on the fluid in response to a voltage applied between at least two of the first plurality of electrodes. In some embodiments of the present invention, a second plurality of electrodes can also be provided in the channel or proximate thereto and spaced across a lateral dimension of the channel to alter the flow of the fluid in the channel (e.g. through an electric field which acts upon different constituents of the fluid differently to separate the constituents in space or time or both).
The first plurality of electrodes are generally substantially planar and oriented in a direction substantially perpendicular to the substrate. The second plurality of electrodes are also generally substantially planar and can be oriented either in a direction substantially perpendicular to the substrate, or in a direction substantially coplanar with the substrate, or both.
Electrical wiring can be formed on the substrate below the first layer of silicon nitride or above the second layer of silicon nitride and connected to the first plurality of electrodes to provide the voltage thereto. Similar wiring can be used to provide electrical connections to the second plurality of electrodes.
As described previously, an entrance port can be provided on one side of the constricted portion of the channel, and an exit port can be provided on the other side of the constricted portion. One or both of the entrance and exit ports can extend through the thickness of the substrate to a back side thereof.
In some embodiments of the present invention, the flow generating means can comprise a coil formed about the channel to produce an electromagnetic field in response to an electrical current flowing through the coil. The coil can be formed from a plurality of turns of an electrical conductor, with each turn further comprising a first portion of the electrical conductor underlying the first layer of silicon nitride, and a second portion of the electrical conductor overlying the second layer of silicon nitride. Electrical wiring can be formed on the substrate below the first layer of silicon nitride or above the second layer of silicon nitride and connected to the coil to provide the electrical current.
To form a hydraulic actuator, a chamber having a deformable or moveable wall can be provided in communication with one end of the channel, with the wall being deformable or moveable in response to a change the flow of the fluid in the channel. The deformable or moveable wall can then be connected to a load using an actuator arm, lever, linkage, compliant mechanism or a combination thereof.
The present invention is further related to a surface-micromachined apparatus, comprising a fluid-flow channel formed on a silicon substrate and lined, at least in part, with silicon nitride; a plurality of vertically-disposed electrical conductors spaced along a portion of the length of the channel either inside or outside of the channel, with the vertically-disposed electrical conductors being oriented substantially perpendicular to the substrate; and electrical wiring formed underneath the channel for electrical activation of the vertically-disposed electrical conductors.
When the vertically-disposed electrical conductors are located inside the channel, these conductors can be electrically activated to produce an electroosmotic force on a fluid within the channel, thereby moving or pressurizing the fluid. To aid in generating the electroosmotic force, a portion of the channel can be constricted with a lateral dimension smaller than the lateral dimension of the remainder of the channel.
When the vertically-disposed electrical conductors are located outside the channel, a plurality of horizontally-disposed electrical conductors can also be provided oriented substantially parallel to the substrate, with each horizontally-disposed electrical conductor being electrically connected to a pair of the vertically-disposed electrical conductors, thereby forming a coil about the channel. Upon electrical activation, the coil produces an electromagnetic field which can act upon a fluid within the channel, thereby moving or pressurizing the fluid or separating particular constituents therein.
Additionally, the present invention relates to a method for forming a fluid-flow channel on a substrate, comprising steps for depositing a first layer of silicon nitride on the substrate; depositing at least one layer of a sacrificial material (e.g. silicon dioxide or a silicate glass) over the first layer of silicon nitride, and patterning the sacrificial material to define a nonuniform shape for the channel, with the nonuniform shape including a constricted portion of the channel which has a height that is smaller than the height of the remainder of the channel; depositing a second layer of silicon nitride over the patterned sacrificial material, with the second layer of silicon nitride conforming to the nonuniform shape of the channel; forming a plurality of vertically-disposed electrical conductors spaced along the length of the constricted portion; and removing the sacrificial material from the channel.
A further step can be provided for forming a plurality of horizontally-disposed electrical conductors, with each horizontally-disposed electrical conductor being electrically connected to a pair of the vertically-disposed electrical conductors, thereby forming a coil.
Another step can be provided for forming electrical wiring below the first layer of silicon nitride, with the electrical wiring being electrically connected to the plurality of vertically-disposed electrical conductors for activation thereof.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.